Near-field communication (NFC) is a short-ranger wireless communication technology for exchanging data between devices over distances of centimeters to tens of centimeters. One example application for NFC is radio-frequency identification (RFID) where a reader device can detect and retrieve data from a tag equipped with an antenna.
NFC devices communicate via magnetic field induction where the two loop antennas are located within each other's near field, effectively forming an air-core transformer.
There are generally two modes of NFC: a passive communication mode where the initiating device provides a carrier field and the target device answers by modulating the existing field; and an active communication mode where both the initiating and target devices communicate by alternately generating their own field.
The target device can be a smartcard or a tag, but also more advanced devices like a mobile phone that can “emulate” the behavior of such a tag. For this reason, the passive communication mode of the target device is usually called “tag emulation mode”. This device can be called “tag emulator” or transponder.
To operate in active mode, both devices typically need to have a power supply, whereas in the passive communication mode two options are possible for the target device: the power may be supplied either by a power source (battery) or from the electromagnetic field provided by the initiating device.
The FIG. 1 illustrates a high-level functional architecture of a tag emulator device TED and of a reader device RD.
The reader device RD comprises a reader transmitter TX_AMP which radiates through the antenna AR a magnetic field. According to the most used standards specifications, the magnetic field is a sinusoidal wave at 13.56 MHz and amplitude between 0.5 and 7.5 Ampere/Meter.
This field is detected by the antenna AT of the tag emulator device TED.
This tag emulator device (or transponder) TED comprises analog functions AF1, AF2, AF3 . . . AFn of an analog area AA, which are connected to the antenna, and a digital area AD.
The first analog function AF1 is a load modulator which receives the analog field signal from the antenna A and modulation data TXMOD from the digital area DA. It modulates the field generated by the reader device RD by changing the load according to the modulation data TXMOD.
By changing the load across the tag emulator antenna AT, the output impedance of the reader antenna AR changes thanks to the electromagnetic coupling between the two antennas. This implies a change in the current flowing through the reader antenna AR and in this way, the modulation data TXMOD is transmitted by load modulation to the reader device RD.
A scaled copy Ic of this current is generated and injected to the analog-to-digital convertor READER_RX, where it is demodulated and processed so as to extract the data MD, which should be identical to the original modulation data TXMOD.
In the tag emulator device TED side, the digital area DA as well as the analog functions AF1, AF2, AF3 . . . AFn of the analog area AA are powered by a power supply S.
As previously said, in a battery mode, the power supply S can be powered by an external supply, for instance the battery of the handset in case the tag emulator device is a mobile terminal.
In a powered-by-the-field (PBF) mode, the power supplied is powered up by harvesting energy from the magnetic field received by the antenna AT.
In general, this harvesting is performed by a rectifier R of a Field Power Supply Unit FPSU, which rectifies the signal so as to provide the supply S with a DC output, called PFF SUPPLY. The power supply S can then generate supply signals S1, S2, S3 . . . Sn for the analog functions AF1, AF2, AF3 . . . AFn respectively and a supply signal Sd for the digital area DA.
More details about the functional architecture depicted in FIG. 1 could be found, for instance, in the book “RFID handbook—Fundamentals and Applications in Contactless Smart Card and Identification”, second edition, Klaus Finkenzeller, Giesecke & Devrient GmBH, Munich, Germany, and especially in section 4.1.10.3 of this book.
However, as explained here-above, during the load modulation, the magnetic field is modulated so that it makes it difficult for the rectifier R to generate a constant DC output PBF_SUPPLY. High load modulation frequency components will appear on this output signal and as a consequence, in the supply signals S1, S2, S3 . . . Sn, Sd generated by the power supply S.
Inconstant supply signal may harm the analog functions AF1, AF2, AF3 . . . AFn as well as the digital area DA.
In some cases where the load modulation index is sufficiently low (usually at longer distance between reader RD and tag emulator devices TED), this issue can be solved by using high-order passive low-pass filter applied on the PBF_SUPPLY signal, but this solution is very costly in terms of silicon area and does not work in all situations, e.g. when the reader device and the tag emulator device are within a closer distance.
There is therefore a need for a solution improving the power supply in the powered-by-the-field (PBF) mode of a tag emulator device.
Also, the field modulation makes possible for spies to get the information transmitted between two NFC devices. A spy coil can be placed between the antennas of the reader and of the tag emulator devices. The coil can read the magnetic signal sent by the reader device RD and then reads the load-modulated signal created by the tag emulator device TED. In this way, it is possible for it to learn everything about the NFC protocol, standard, signal quality, signal level, signal timing, data sent and data received.
There is also a need to avoid such spy coils to get the transmitted information between the two devices.